1. Field of the Invention
This invention relates to a semiconductor device, and more particularly to a power semiconductor device having a superjunction structure.
2. Background Art
Power semiconductor devices such as power MOSFET (metal oxide semiconductor field effect transistor) and IGBT (insulated gate bipolar transistor) are widely used for electric power conversion and power control in home electric apparatuses, communication apparatuses, and car-mounted motors. These power semiconductor devices have fast switching characteristics and a reverse device voltage (breakdown voltage) of several ten to several hundred volts. Semiconductor devices having optimal breakdown voltage are selected in accordance with applications.
Recently, it has been desired to downsize electric apparatuses and to increase their efficiency. With regard to the power semiconductor device incorporated in such electric apparatuses, there has been a strong demand for reducing the resistance during the ON state of the semiconductor device (ON resistance) while retaining high breakdown voltage. Typically, in a power semiconductor device, a p-type base region is connected to a source electrode, and an n-type drift region is connected to a drain electrode. In the OFF state of the semiconductor device, when high voltage is applied between the drain electrode and the source electrode, a depletion layer extends from the pn junction interface between the p-type base region and the n-type drift region (first pn junction interface) into the drift region and serves to sustain the voltage. At this time, the breakdown voltage is determined by the distance of the extending depletion layer, which depends on the dopant concentration in the drift region.
However, electric field strength in the semiconductor device is maximized at the first pn junction interface. As the dopant concentration in the drift region increases, the strength of electric field applied to the first pn junction interface increases. Thus the dopant concentration in the drift region has a certain limit determined by the breakdown voltage of the first pn junction interface. Hence, to obtain high breakdown voltage, the dopant concentration in the drift region needs to be decreased. However, this results in increasing the resistance of the drift region, and increasing the proportion of the resistance of the drift region accounting for the ON resistance of the overall semiconductor device. Consequently, the ON resistance of the overall semiconductor device increases.
Thus there is a tradeoff between the breakdown voltage and the ON resistance of the semiconductor device, and the increase of breakdown voltage results in the increase of ON resistance. The relationship between breakdown voltage and ON resistance is determined by the material forming the semiconductor device. It is extremely difficult to simultaneously realize high breakdown voltage and low ON resistance beyond the theoretical limit without improving the structure of the semiconductor device.
Hence a structure is proposed for decreasing the resistance of the drift region beyond the theoretical limit of the material (see, e.g., T. Fujihira, “Theory of semiconductor superjunction devices”, Jpn. J. Appl. Phys., Vol. 36 (1997), pp. 6254-6262, hereinafter referred to as Non-Patent Document 1). Non-Patent Document 1 discloses a structure having a drift region where p-type semiconductor layers and n-type semiconductor layers are alternately arranged along the direction orthogonal to the direction of current flow. In such a structure, a depletion layer extends also from the pn junction interface between the p-type semiconductor layer and the n-type semiconductor layer in the drift region (second pn junction interface), besides the first pn junction interface described above. Thus the electric field is prevented from concentrating only on the first pn junction interface. Hence sufficient breakdown voltage can be sustained even if the dopant concentration in the n-type semiconductor layer is made higher than in the drift region of a normal semiconductor device. Consequently, the ON resistance of the semiconductor device, particularly the drift resistance, can be reduced.
The semiconductor device disclosed in Non-Patent Document 1 is known as a lateral semiconductor device because current flows laterally in the semiconductor device. In contrast, there is also a disclosure of a vertical semiconductor device, where the p-type semiconductor layers and n-type semiconductor layers in the drift region are vertically arranged, passing current vertically (see, e.g., JP-A 2001-244461(Kokai)). Such a structure of the drift region is known as a superjunction structure.
In a semiconductor device having a superjunction structure, it is a major problem how to ensure sufficient breakdown voltage in the edge termination section, which is the peripheral section of the semiconductor device. If the breakdown voltage of the edge termination section is low, the breakdown voltage of the overall semiconductor device decreases because it is determined by the breakdown voltage of the edge termination section. Furthermore, it also raises concerns about the decrease of reliability of the semiconductor device. Moreover, avalanche current concentrating on the edge termination section also causes the breakdown of the semiconductor device.